HUBBLE IDENTIFIES WHAT MAY BE THE MOST LUMINOUS STAR KNOWN

Astronomers using NASA's Hubble Space Telescope have identified
what may be the most luminous star known -- a celestial mammoth
which releases up to 10 million times the power of the Sun and is
big enough to fill the diameter of Earth's orbit. The star
unleashes as much energy in six seconds as our Sun does in one
year.

The image, taken by a University of California, Los Angeles
(UCLA)-led team with the recently installed Near-Infrared Camera
and Multi-Object Spectrometer (NICMOS) aboard Hubble, also
reveals a bright nebula, created by extremely massive stellar
eruptions. The nebula is so big (four light-years in diameter)
that it would nearly span the distance from the Sun to Alpha
Centauri, the nearest star to Earth's solar system.

The astronomers estimate that when the titanic star was formed
one to three million years ago, it may have weighed up to 200
times the mass of the Sun before shedding much of its mass in
violent eruptions.

"This star may have been more massive than any other star, and
now it is without question still among the most massive -- even
at the low end of our estimates," says Don F. Figer of UCLA.
"Its formation and life stages will provide important tests for
new theories about star birth and evolution."

Violent Eruptions Produce Nebula

The UCLA astronomers estimate that the star, called the "Pistol
Star" (for the pistol shaped nebula surrounding it), is
approximately 25,000 light-years from Earth near the center of
our Milky Way galaxy. The Pistol Star is not visible to the eye,
but is located in the direction of the constellation Sagittarius,
hidden behind the great dust clouds along the Milky Way.

The Pistol Star was first noted in the early 1990s by astronomers
in South Africa and Japan, but its luminosity and relationship to
the nebula was not realized until 1995, when Figer proposed in
his Ph.D. thesis that the "past eruptive stages of the star"
might have created the nebula. The Hubble spectrometer results
confirm this conclusion.

The astronomers believe that the Pistol nebula was created by
eruptions in the outer layers of the star which ejected up to 10
solar masses of material in giant outbursts about 4,000 and 6,000
years ago. The star will continue to lose more material,
eventually revealing its bare hot core, sizzling at 100,000
degrees.

Burning at such a dramatic rate, the Pistol Star is destined for
certain death in a brilliant supernova in 1-3 million years.
"Massive stars are burning their candles at both ends; they are
so luminous that they consume their fuel at an outrageous rate,
burning out quickly and often creating dramatic events, such as
exploding as supernovae," said Mark Morris, a UCLA professor
of astronomy and co-investigator. "As these stars evolve, they
can eject substantial portions of their atmospheres -- in the
case of the Pistol Star, producing the nebula and an extreme
stellar wind (outflow of charged particles) that is 10 billion
times stronger than our Sun's."

Why Has it Taken So Long to Find?

The Pistol Star would be visible to the naked eye as a fourth
magnitude star in the sky (which is quite impressive given its
distance of 25,000 light-years) if it were not for interstellar
dust clouds of tiny particles between the Earth and the center of
the Milky Way that absorb the star's light. The most powerful
telescopes cannot see the Pistol Star in visible wavelengths.
However, ten percent of the infrared light leaving the Pistol
Star reaches Earth, putting it within reach of infrared
telescopes, which have seen rapid technological advances in
recent years -- spurred by projects such as NICMOS.

What Are the Implications?

The Pistol Star was so massive when it was born that it brings
into question current thinking about how stars are formed, say
the UCLA astronomers. In the current view, stars form within
large dust clouds which contract under their own gravity,
eventually forming hot clumps that ignite the hydrogen fusion
process.

The star may radiate enough energy to halt the inward fall of
material, thus limiting its maximum mass. The initial mass of
the Pistol Star may have exceeded this theoretical upper limit.
"It is perhaps no accident that this extreme-mass star is found
near the center of the Galaxy," says Morris. "Current evidence
leads us to believe that the star formation process there may
favor stars much more massive than our modest Sun."

Over the coming year, the team will be using the new
near-infrared spectrometer that Ian S. McLean's team is building
at UCLA for the giant 10-meter Keck II telescope in Hawaii. The
new instrument will be used to measure the velocities of the
expanding gas shells.

In addition to Figer, Morris, and McLean, the team also includes
Caltech physicist Gene Serabyn and Columbia University astronomer
R. Michael Rich.

The Space Telescope Science Institute is operated by the
Association of Universities for Research in Astronomy, Inc.
(AURA), for NASA, under contract with the Goddard Space Flight
Center, Greenbelt, MD. The Hubble Space Telescope is a project
of international cooperation between NASA and the European Space
Agency.

The Space Telescope Science Institute is operated by the
Association of Universities for Research in Astronomy, Inc.
(AURA), for NASA, under contract with the Goddard Space Flight
Center, Greenbelt, MD. The Hubble Space Telescope is a project
of international cooperation between NASA and the European Space
Agency (ESA).

HUBBLE UNCOVERS BRILLIANT STAR IN MILKY WAY'S CORE

One of the intrinsically brightest stars in our galaxy appears as
the bright white dot in the center of this image taken with
NASA's Hubble Space Telescope. Hubble's Near Infrared Camera
and Multi-Object Spectrometer (NICMOS) was needed to take the
picture, because the star is hidden at the galactic center,
behind obscuring dust. NICMOS' infrared vision penetrated
the dust to reveal the star, which is glowing with the radiance
of 10 million suns.

The image also shows one of the most massive stellar eruptions
ever seen in space. The radiant star has enough raw power to
blow off two expanding shells (magenta) of gas equal to the mass
of several of our suns. The largest shell is so big (4
light-years) it would stretch nearly all the way from our Sun to
the next nearest star. The outbursts seen by Hubble are
estimated to be only 4,000 and 6,000 years old, respectively.

Despite such a tremendous mass loss, astronomers estimate the
extraordinary star may presently be 100 times more massive than
our Sun, and may have started with as much as 200 solar masses of
material, but it is violently shedding much of its mass.

The star is 25,000 light-years away in the direction of the
constellation Sagittarius. Despite its great distance, the star
would be visible to the naked eye as a modest 4th magnitude
object if it were not for the dust between it and the Earth.

This false-colored image is a composite of two separately
filtered images taken with the NICMOS, on September 13,1997.
The field of view is 4.8 light-years across, at the star's
distance of 25,000 light-years. Resolution is 0.075 arc seconds
per pixel (picture element).